Glacial landforms and features

Photo by: pablo_hernan

During
the last Ice Age, which ended approximately 10,000 years ago, 32 percent
of Earth's land area was covered with glaciers. At present,
glaciers cover roughly 10 percent of the land area. A vast majority of
that glacial ice overlies much of the continent of Antarctica. Most of the
rest covers a great portion of Greenland; a small percentage is found in
places such as Alaska, the Canadian Arctic, Patagonia, New Zealand, the
Himalayan Mountains, and the Alps.

Glaciers are not landforms. The action of glaciers, however, creates
landforms. It is a process known as glaciation. Glacial ice is an active
agent of erosion, which is the gradual wearing away of Earth surfaces
through the action of wind and water. Glaciers move, and as they do, they
scour the landscape, "carving" out landforms. They also
deposit rocky material they have picked up, creating even more features.
The work of present-day glaciers, however, is slow and confined to certain
areas of the planet. Less obvious but far more reaching has been the work
of Ice Age glaciers. Many of the distinctive features of the northern
landscapes of North America and Europe were formed by glaciers that once
covered almost one-third of the planet's land surface.

The shape of the land

A glacier is a large body of ice that formed on land from the compaction
and recrystallization of snow, survives year to year, and shows some sign
of movement downhill due to gravity. Two types of glaciers exist:
relatively small glaciers that form in high elevations near the tops of
mountains are called alpine or mountain glaciers; glaciers that form over
large areas of continents close to the poles (the North and South Poles;
the extreme northernmost and southernmost points on the globe) are called
continental glaciers or ice sheets. Two continental glaciers are found on
Earth: one covers 85 percent of Greenland in the Northern
Hemisphere and the other covers more than 95 percent of Antarctica in the
Southern Hemisphere.

Glacial landforms and features: Words to Know

Ablation zone:

The area of a glacier where mass is lost through melting or
evaporation at a greater rate than snow and ice accumulate.

Accumulation zone:

The area of a glacier where mass is increased through snowfall at a
greater rate than snow and ice is lost through ablation.

Alpine glacier:

A relatively small glacier that forms in high elevations near the tops
of mountains.

Arête:

A sharp-edged ridge of rock formed between adjacent cirque glaciers.

Basal sliding:

The sliding of a glacier over the ground on a layer of water.

Cirque:

A bowl-shaped depression carved out of a mountain by an alpine
glacier.

Continental glacier:

A glacier that forms over large areas of continents close to the
poles.

Crevasse:

A deep, nearly vertical crack that develops in the upper portion of
glacier ice.

Erosion:

The gradual wearing away of Earth surfaces through the action of wind
and water.

Erratic:

A large boulder that a glacier deposits on a surface made of different
rock.

Esker:

A long, snakelike ridge of sediment deposited by a stream that ran
under or within a glacier.

Firn:

The granular ice formed by the recrystallization of snow; also known
as névé.

Fjord:

A deep glacial trough submerged with seawater.

Glacial drift:

A general term for all material transported and deposited directly by
or from glacial ice.

Glacial polish:

The smooth and shiny surfaces that are produced on rocks underneath a
glacier by material carried in the base of that glacier.

Glacial surge:

The rapid forward movement of a glacier.

Glacial trough:

A U-shaped valley carved out of a V-shaped stream valley by the
movement of a valley glacier.

Glaciation:

The transformation of the landscape through the action of glaciers.

Glacier:

A large body of ice that formed on land by the compaction and
recrystallization of snow, survives year to year, and shows some sign
of movement downhill due to gravity.

A shallow glacial trough that leads into the side of a larger, main
glacial trough.

Horn:

A high mountain peak that forms when the walls of three or more
glacial cirques intersect.

Internal flow:

The movement of ice inside a glacier through the deformation and
realignment of ice crystals; also known as creep.

Kame:

A steep-sided, conical mound or hill formed of glacial drift that is
created when sediment is washed into a depression on the top surface
of a glacier and then deposited on the ground below when the glacier
melts away.

Kettle:

A shallow, bowl-shaped depression formed when a large block of glacial
ice breaks away from the main glacier and is buried beneath glacial
till, then melts. If the depression fills with water, it is known as a
kettle lake.

Lateral moraine:

A moraine deposited along the side of a valley glacier.

Medial moraine:

A moraine formed when two adjacent glaciers flow into each other and
their lateral moraines are caught in the middle of the joined glacier.

Meltwater:

The water from melted snow or ice.

Moraine:

A general term for a ridge or mound of till deposited by a glacier.

Piedmont glacier:

A valley glacier that flows out of a mountainous area onto a gentle
slope or plain and spreads out over the surrounding terrain.

Rock flour:

Fine-grained rock material produced when a glacier abrades or scrapes
rock beneath it.

Snow line:

The elevation above which snow can form and remain all year.

Striations:

The long, parallel scratches and grooves produced in rocks underneath
a glacier as it moves over them.

Tarn:

A small lake that fills the central depression in a cirque.

Terminal moraine:

A moraine found near the terminus of a glacier; also known as an end
moraine.

Terminus:

The leading edge of a glacier; also known as the glacier snout.

Till:

A random mixture of finely crushed rock, sand, pebbles, and boulders
deposited by a glacier.

Valley glacier:

An alpine glacier flowing downward through a preexisting stream
valley.

Both types of glaciers create landforms through erosion. These erosional
features may be as large as the Great Lakes of North America or as small
as scratches left in pebbles. As a glacier moves, it scours away material
underneath it, plucking up rocks, some of which may be house-sized
boulders. This material then becomes embedded in the ice at the base of a
glacier. As the glacier continues to move, the embedded material abrades
or scrapes the rock underneath. The slow scraping and grinding produces a
fine-grained material known as rock flour. It also produces long parallel
scratches and grooves known as striations in the underlying rocks. Because
they are aligned parallel
to the direction of ice flow, glacial striations help geologists
determine the flow path of former glaciers. Another small-scale erosional
feature is glacial polish. This is a smooth and shiny surface produced on
rocks underneath a glacier when material encased in the ice abrades the
rocks like fine sandpaper.

Moving ice sculpts a variety of landforms out of the landscape.
Larger-scale erosional features include bowl-shaped, steep-walled
depressions carved out of the side of mountains. These depressions are
called cirques (pronounced SIRKS), and the relatively small alpine
glaciers that fill them are called cirque glaciers. If the glacier melts
and a small lake fills the central depression in a cirque, that lake is
known as a tarn. Two or more glacial cirques may form on a mountainside,
eroding away the rock
between them to create a steep-sided, sharp-edged ridge known as an
arête (pronounced ah-RHET). When the walls of three or more glacial
cirques meet, they may form a high mountain peak known as a horn.

When a cirque glacier expands outward and flows downward through a stream
valley that already exists, it becomes a valley glacier. Through erosion,
valley glaciers turn V-shaped stream valleys into U-shaped glacial
troughs. Smaller valley glaciers, known as tributary glaciers, may form
alongside a main valley glacier and eventually flow into it. The shallower
glacial troughs created by these glaciers are known as hanging valleys. A
valley glacier that flows out of a mountainous area onto a gentle slope or
plain and spreads out over the surrounding terrain is a piedmont glacier.
A valley glacier may flow all the way to a coastline, carving out a narrow
glacial trough. If the glacier melts and the valley fills with seawater,
it is known as a fjord (pronounced fee-ORD). Although prominent along the
west coast of Norway, fjords are also found along the coasts of Alaska,
British Columbia, Chile, Greenland, New Zealand, and Scotland.

Glaciers leave their mark on the landscape not only through erosion, but
also through deposition. Deposition involves carrying loose materials from
one area and leaving, or depositing, these materials in another area.
Depositional features are created by the release of rocky material from a
glacier. They vary widely in scale and form. All sediment (rock debris
ranging from clay to boulders) deposited as a result of glacial erosion is
called glacial drift. Like a stream, a glacier picks up and carries
sediment particles of various sizes. Unlike a stream, a glacier can carry
part of that sediment load on its bottom, its sides, or its top (sediment
on top has fallen onto the glacier from the valley walls). Another
difference between the two is that when a stream deposits its load of
sediment, it does so in order of size and weight: large, heavy particles
are deposited first, followed by particles that are increasingly smaller
and lighter. When a glacier deposits sediment, there is no such order. The
particles are unsorted, with large and small particles mixed together.
This random mixture of finely crushed rock, sand, pebbles, and boulders
deposited by a glacier is referred to as till.

Since a glacier can carry rocks for great distances before depositing
them, those rocks generally differ from the surrounding native rocks in
that area. In fact, because they are derived from a very large area eroded
by a glacier, glacial deposits contain the widest variety of rock types. A
glacially deposited large boulder that differs in composition from the
rocks around it is called an erratic.

A deposit of till that forms a ridge or mound is called a moraine
(meh-RAIN). Moraines deposited along the sides of alpine glaciers are
called lateral moraines. When two valley glaciers converge to create a
single larger glacier, their opposing lateral moraines merge to form a
ridge that

Major features of glaciation, or the action of glaciers on a
landscape
.

runs down the middle of the new glacier. This is a medial moraine. A
moraine deposited at the leading edge of a glacier, marking its farthest
advance, is a terminal or end moraine. Finally, a continuous layer of till
deposited beneath a steadily retreating glacier is a ground moraine.

Another common glacial landform is the drumlin. This tear-drop-shaped hill
forms underneath a glacier. The tail of the drumlin points in the
direction of the ice movement. Geologists are unsure exactly how drumlins
form, whether a glacier scrapes up material beneath it or deposits
material it already carries or a combination of both. Drumlins may be
quite large, measuring up to 200 feet (60 meters) in height and 0.6 mile
(1 kilometer) in length.

As a glacier melts, it produces meltwater that flows on top, within, and
underneath the glacier through channels. This meltwater moves large
quantities of sediment from the glacier. At the leading edge of the
glacier, also known as the terminus or glacier snout, the meltwater
emerges in large streams that carry it away from the glacier. The sediment
in the meltwater is then deposited, forming a broad, sweeping plain called
an outwash plain. Since the sediment was carried in water, it is deposited
in a sorted manner, with the largest particles first and the smallest
particles last. If a glacier melts and retreats, curving, snakelike ridges
of sediment may mark the former locations of streams that existed under
the glacier. These long, twisting ridges are called eskers.

Two other features that result from the melting of glaciers are kames and
kettles. As a glacier begins to melt, a depression may form on its top
surface, filling with water and sediment. When the glacier finally melts
away, the sediment is set down on the surface of the ground, forming a
steep-sided, conical mound or hill known as a kame. A kettle forms when a
large chunk of ice separates from the main glacier. Buried by glacial
till, the ice then melts, leaving a depression in the landscape. This
eventually becomes filled with water, forming a kettle lake.

Forces and changes: Construction and destruction

Glaciers are moving ice. They can range in size from small patches to ice
sheets covering millions of square miles. The world's largest
alpine glacier is the Siachen Glacier in the Karakoram Mountains of
Pakistan. Measuring 47 miles (75 kilometers) in length, it contains more
than 13.6 cubic miles (56.7 cubic kilometers) of ice. The Antarctic ice
sheet is the largest single mass of ice on Earth. It covers an area of
almost 5.4 million square miles (14 million square kilometers) and
contains over 7 million cubic miles (29 million cubic kilometers) of ice.

Loess

Loess (pronounced LUSS; a German word meaning "loose") is
a deposit of fine, yellowish-gray, silty sediment. Composed of mineral
particles finer than sand but coarser than dust or clay, loess forms
fertile topsoils. Areas with large loess deposits are found in the
central and northwestern United States, in central and eastern Europe,
and in eastern China.

The majority of loess was formed by the action of glaciers and wind
(some loess comes from the transport of sediment from desert areas).
After the last ice age, meltwater streams from the retreating
continental glaciers transported vast amounts of rock flour and other
fine sediment away from the glaciers. Strong winds blowing off the
glaciers (because glacial ice cools the air and cold air moves to lower
elevations at the front of the glacier) picked up the fine sediment and
carried it far beyond the outwash plains before it was deposited.

Since loess is transported in the air, it is very well sorted, and is
mostly silt combined with a small amount of clay. Loess is generally
deposited as a blanket over everything, both hills and valleys. It is
often removed by wind and water to fill up basins and depressions.

Glacial formation

A glacier does not start out as a glacier. All that ice began to form when
snow—delicate, feathery crystals of ice—fell in areas above
the snow line, the elevation above which snow can form and remain all
year. It takes snow on top of snow on top of more snow to create a
glacier; it also takes a long time. On average, 10 feet (3 meters) of snow
will turn into 1 foot (0.3 meter) of ice. In polar regions, where annual
snowfall is

Glacial effects and features
.

generally very low because the air is too cold to hold much moisture, it
may take snow about 1,000 years to turn into ice.

In time, if snow does not melt but is buried beneath additional layers of
snow, it will begin to compress. This forces the snow crystals to
recrystallize,
forming grains similar in size and shape to cane sugar. As new snow piles
on top and the snow below becomes further compressed, the grains grow
larger and the air spaces between them become smaller. Over a short period
of time, perhaps the span of two winters, the compressed snow turns into a
granular material known as firn or névé (pronounced
nay-VAY). The density (amount of mass in a given volume) of regular snow
is about 10 percent that of water. The density of firn is about 50 percent
that of water. Once the thickness of the overlying snow exceeds about 165
feet (50 meters), the firn turns into a solid mass of glacial ice.

Additions to a glacier's mass are called accumulation; losses
through melting, erosion, or evaporation are called ablation (pronounced
ah-BLAY-shun). A glacier may be divided into two distinct zones. Where
snow and ice accumulate faster than they melt away or evaporate is the
accumulation zone; where melting and evaporation occur faster than
accumulation is the ablation zone. The upper part of a glacier is its
accumulation zone, while the lower part is its ablation zone. The boundary
between the two zones is called the firn limit.

Over a period of years, depending on the amount of snowfall and seasonal
temperatures, a glacier may gain more mass than it loses. If this occurs,
the terminus of the glacier will likely advance. If the opposite happens,
with the glacier losing more mass than it gains, its terminus will likely
retreat. Thus, depending on the balance between accumulation and ablation,
a glacier may grow or shrink.

Ice flow

A glacier always moves in the same direction whether it is advancing or
retreating. It moves to lower elevations under the force of gravity by two
different processes: internal flow and basal sliding. The glacial ice
beneath the firn in a glacier is so dense and under such pressure that it
begins to behave like thick tar or what geologists term
"plastic." The individual ice crystals in this area respond
to pressure and the force of gravity by deforming yet again. They are
forced into the same orientation or direction, all realigning parallel to
the direction of flow. Like cards in a deck of playing cards, they then
slide over and past one another. Glacial movement through internal flow,
also known as creep, is very slow: on average, it measures only an inch or
two (a few centimeters) a day. In a valley glacier, ice in the upper
central part moves faster than ice at the sides, where it is in contact
with the valley walls.

Confined by high pressures, ice deep in a glacier does not crack during
internal flow. However, near the surface of the glacier where there is
less pressure, the ice is brittle. When the lower portion of a glacier
moves by internal flow, especially over abrupt changes in slope, large
cracks may
develop in the upper 150 feet (45 meters) or so of ice. These deep,
nearly vertical cracks are called crevasses (pronounced kri-VASS-ez).

Glaciers in polar regions are frozen to the ground and move only through
internal flow. Glaciers elsewhere are normally warm enough at their bases
to have a layer of water form between their ice and the ground. The water
reduces friction by lubricating the ground and allowing the glacier to
slide on its bed in what is called basal sliding. This second type of
glacial movement occurs because high pressure reduces the temperature at
which ice will melt. Ice underneath a 7,220-foot (2,200-meter) glacier
will melt at roughly 29°F (–1.6°C), rather than at
32°F (0°C). The thicker the glacier, the greater the
pressure at its base, and the lower the temperature at which its ice will
melt.

Ice Ages

Ice ages were periods in Earth's history when vast glaciers
covered large portions of the planet's surface. Earth's
average annual temperature varies constantly from year to year, from
decade to decade, and from century to century. During some periods, that
average annual temperature has dropped low enough to allow fields of ice
to grow and cover large areas of Earth. Annual variations of only a few
degrees can result in the formation of extensive continental glaciers.

Over the last 2.5 million years, about twenty-four ice ages have
occurred. This means that Earth's average annual temperature
shifted upwards and downwards about two dozen times during that period.
In each case, an episode of significant cooling was followed by an
episode of significant warming, called an interglacial period, after
which cooling took place once more. At present, Earth is in an
interglacial period.

The exact causes for ice ages have not been proven. Scientists believe
that ice ages are the result of a complicated interaction between such
things as variations in the Sun's energy output, the varying
distance of Earth from the Sun, variations in the tilt of Earth's
axis, the changing position and height of the continents, changing
oceanic circulation, and changes in the composition of the atmosphere.

Other factors may also contribute to basal sliding. Because ice acts like
a blanket, a glacier traps heat that escapes from the surface of Earth.
Although not much, this heat may be enough to raise the temperature of ice
at the base of a glacier to a little above the pressure-melting point.
Meltwater from the top or inside a glacier may also make its way down
through cracks and channels to the glacier's base, contributing to
the layer of water formed there. Glacial movement due to basal sliding may
be ten times faster than that due to internal flow. Because of this, basal
sliding plays an
important role in how much a glacier erodes a landscape and creates
landforms.

On rare occasions, an alpine glacier may unexpectedly surge downslope,
moving at a rate of 165 to 330 feet (50 to 100 meters) per day. This
results in a jumbled mass of ice along the terminus of the glacier and
many crevasses along its top. Although geologists do not completely
understand the reasons for glacial surging, they believe it may be caused
by a buildup of meltwater at the base of a glacier that reduces the normal
friction and allows unusually fast basal sliding. The fastest-recorded
glacial surge was that of the Kutiah Glacier in northern Pakistan. Over a
three-month period in 1953, the glacier slid more than 7.5 miles (12
kilometers), averaging about 367 feet (112 meters) per day.

Through the combination of internal flow and basal sliding, glaciers move
over a landscape, scraping and plucking the rock surfaces over which they
move. They transport unsorted sediment both internally and on their
surfaces. During warmer periods, a glacier may lose part of its mass, its
ice turning to meltwater, which carries sediment away from the terminus of
the glacier. Even as a glacier's terminus retreats, the flow of ice
in the glacier continues to move downward under the influence of gravity.

Scientific measurements at the beginning of the twenty-first century
showed that most glaciers worldwide were retreating. Glaciers in the
Himalayan Mountains were wasting away the quickest. Scientists who
conducted the research found a connection between increasing temperatures
around the world and the glacial retreat. It is known that over the last
100 years, global sea levels have risen 4 to 10 inches (10 to 25
centimeters). Scientists estimate that the melting of glaciers has
contributed 1 to 2 inches (2.5 to 5 centimeters) to that rise.

The Literary Landscape

"These islands of ice and black basalt, now and then tinged
russet or blue by oozings of iron or copper, rise over 600 meters. Their
hearts are locked under deep glaciers, a crystal desert forever frozen
in terms of our short life spans, but transient in their own time scale.
Sometimes one sees only the cloud-marbled glacial fields, high in the
sun above hidden mountain slopes and sea fog, Elysian plains that seem
as insubstantial as vapor. The interiors of the glaciers, glimpsed
through crevasses, are neon blue. Sliding imperceptibly on their
bellies, the glaciers carve their own valleys through the rock, and when
they pass over rough terrain they have the appearance of frozen rapids,
which is in fact what they are, cascading at a rate of a centimeter a
day."

—David C. Campbell,
The Crystal Desert: Summers in Antarctica
, 1992.

Spotlight on famous forms

Glacier National Park, Montana

Located in northwestern Montana on the border between the United States
and Canada are 1,013,572 acres (410,497 hectares) of pristine

Map of glaciers around the world. Glaciers cover roughly 10 percent
of Earth's land area. A vast majority of that, 90 percent,
overlies the continent of Antarctica
.

wilderness. Glacier National Park, established in 1910 as the
country's tenth national park, contains some fifty glaciers and
more than two hundred glacier-fed lakes. The valleys and other geologic
features of the park were all eroded and carved by the action of glaciers
over the last two billion years. Several times over the past two million
years, huge glaciers carved the mountains and valleys and then retreated,
leaving a newly sculpted landscape. The most recent continental glacier
that covered the upper section of North America retreated over ten
thousand years ago. The fifty alpine glaciers in the park formed during
the last few thousand years.

The park is filled with many glacial features: arêtes, cirques,
hanging valleys, horns, and moraines. Among the more famous ones are Mount
Reynolds, a glacial horn; Garden Wall, a towering arête that
extends for miles; and the U-shaped St. Mary Valley.

Matterhorn, Switzerland

One of the most recognizable mountains in the world, the Matterhorn in the
Pennine Alps on the border between Switzerland and Italy rises some 14,700
feet (4,480 meters). First successfully climbed in 1865, it is celebrated
for its distinctive shape. The mountain is a classic example of
a horn. Eroded by cirques, its steep sides meet in arêtes that
lead to the hornlike, pointed peak.

The Alps mountain system in southern-central Europe curves in a great arc
for approximately 500 miles (800 kilometers). It runs from the
Mediterranean Sea up along the borders and adjacent regions of France,
Italy, Switzerland, Germany, and Austria, before ending in Slovenia. The
Alps was the first mountain system to be studied extensively by
geologists. Many of the geologic terms associated with mountains and
glaciers originated in those studies.

Walden Pond, Massachusetts

Walden Pond, the deepest lake in Massachusetts, lies in the northeast part
of the state near the city of Concord. It is a kettle lake, created about
ten thousand years ago when continental ice from the last ice age began to
retreat. A huge block of that glacial ice broke off and remained behind,
surrounded at it base by sand and gravel deposited by meltwater streams.
The block melted over a period of about two hundred years, forming a
steep-sided basin that filled with water. The current shape of the pond,
with its steep sides, coves along its margins, and two deep areas,
reflects the shape of the original block of ice. The current maximum depth
of the pond is 103 feet (31 meters). The clear water that fills the lake
comes from precipitation and groundwater (freshwater lying within the
uppermost parts of Earth's crust, filling the pore spaces in soil
and fractured rock).

The lake is famous because American writer Henry David Thoreau
(1817–1862) lived along its shores between 1845 and 1847. While
there, he wrote
Walden, or Life in the Woods
(published in 1854). In this work, a series of essays, Thoreau combined
writing on transcendental philosophy with observations of aquatic ecology
and aspects of limnology, the study of lakes. He also championed the value
of living close to nature. Because of this highly influential work, many
people consider Walden Pond and the area around it to be the birthplace of
the American conservation movement.

In 1965, the U.S. National Park Service designated Walden Pond as a
National Historic Landmark.

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